Method for pressure infiltration casting using a vent tube

ABSTRACT

A method for pressure infiltration casting is provided wherein steps of preheating and evacuating a mold cavity and infiltrant charge are carried out in a separate vessel from a pressure vessel wherein the mold cavity is filled using a vent tube, allowing for rapid finished article throughput.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and apparatus for pressureinfiltration casting.

2. Description of the Prior Art

In currently used pressure infiltration processes, as described by U.S.Pat. Nos. 5,111,870 and 5,111,871 to Cook and in general reviews of thestate of the pressure infiltration casting art such as Cook et al.,"Pressure infiltration casting of metal matrix composites", MaterialsScience and Engineering, A144, (1991) pages 189-206, a cold moldcontaining a preform is loaded into the tooling which serves as acombined pressure vessel/vacuum furnace. A charge of solid infiltrantwhich can be metal is placed on top of the preform and is separated fromthe preform by a filter. The filter is characterized by sufficiently lowpermeability and lack of wetting with the liquid infiltrant to preventpremature infiltrant penetration into the preform and chemical inertnesswith respect to the infiltrant to avoid contamination of the infiltrant.The filter material also acts as a thermal insulator so that infiltrantcharge temperature and preform temperature can be independentlycontrolled.

Next, the preform is heated and the solid infiltrant charge is meltedunder vacuum in the pressure vessel/vacuum furnace. Since the infiltrantis melted in a vacuum and the mold is not gas permeable, a vacuum isisolated in the preform contained in the mold cavity.

Then, the pressure vessel/vacuum furnace is pressurized to create apressure gradient between the pressurized mold exterior and the vacuumisolated in the preform contained within the mold interior. It is thispressure differential that drives the infiltration process.

After infiltration is complete, the final step of the process issolidification of the infiltrated preform. Solidification of theinfiltrated preform is also conducted within the pressure vessel/vacuumfurnace by providing a temperature gradient appropriate to result indirectional solidification. Several techniques to obtain directionalsolidification in a pressure infiltration process are known in the artincluding lowering of the infiltrated preform into a "chill zone" asdemonstrated by Klier et al., "Fabrication of cast particle-reinforcedmetals via pressure infiltration", Journal of Materials Science, 26,(1991), pages 2519-2526 or, alternatively, lifting a cooled chill deviceto contact the preform as described in U.S. Pat. Nos. 5,111,870 and5,111,871 to Cook. During directional solidification in a pressureinfiltration process, liquid infiltrant in the hot zone of theinfiltrated preform solidifies last and serves as a sprue and reservoirfor feeding porosity as the rest of the infiltrated preform solidifies.

The three steps involved in the foregoing prior art pressureinfiltration processes, preform and infiltrant charge heating andevacuation, preform infiltration, and infiltrated preformsolidification, each take different amounts of time. Preform andinfiltrant charge heating and evacuation take the longest amount oftime, infiltrated preform solidification takes less time than preformand infiltrant charge heating and evacuation and pressure infiltrationof the preform takes the least time. For example, using a two inch byfour inch by eight inch mold cavity, 600 grams of aluminum infiltrantand a silicon carbide particulate preform as would typically beencountered in use of prior art pressure infiltration methods, in therange of from about 2 to about 3 hours are needed to preheat the preformand melt the aluminum infiltrant charge under vacuum, less than about 1minute is required to infiltrate the heated preform with the moltenaluminum infiltrant and less than about 6 minutes are needed to cool themold to a temperature less than the solidus temperature of the aluminuminfiltrant. Once the mold is removed from the pressure vessel/vacuumfurnace, the pressure vessel/vacuum furnace can be used to resume thethree step pressure infiltration process.

While the foregoing prior art pressure infiltration process is a highlyeffective and controllable process, the throughput of finished pressureinfiltrated articles is inherently limited by the slowest step of thepressure infiltration process, that of heating the preform and meltingthe infiltrant charge, which as demonstrated by the foregoing example,is as long as 3 hours by comparison with a total of at most 16 minutesfor the other two steps of the process, preform infiltration andinfiltrated preform solidification combined. Although the pressurevessel/vacuum furnace pressure infiltration capability is only neededduring the two shortest steps of the pressure infiltration process, thistooling is in constant use, even for the most time consuming steps ofthe process, because it is also used for preform and infiltrant heatingand evacuation.

Thus, according to existing pressure infiltration techniques, preformand infiltrant heating and evacuation as well as pressure infiltrationare performed sequentially in the same pressure vessel/vacuum furnacetooling, thus occupying this multipurpose tooling for all three stagesof the pressure infiltration casting process, when, in fact, thepressure vessel function of the tooling is only required for the rapidlyaccomplished step of pressure infiltration and solidification. Theseexisting pressure infiltration processes are limited by their sloweststep, preform and infiltrant heating.

Thus, there exists a need for a rapid and economical pressureinfiltration process wherein the throughput of finished articles islimited only by the solidification rate of the infiltrated mold cavityand wherein the steps of mold cavity and infiltrant heating andevacuation are performed in separate apparatus from the steps of moldcavity infiltration and infiltrated mold cavity solidification.

SUMMARY OF THE INVENTION

The invention provides a rapid and economical pressure infiltrationprocess which operates at the fundamental limit of processing time andfinished article throughput, the filled mold/infiltrated preformsolidification rate. Such rapid throughput is achieved by heating andevacuating the mold cavity, which can contain a preform, and theinfiltrant in furnaces and/or vacuum furnaces which are separate fromthe pressure vessel wherein mold cavity filling and filled mold cavitysolidification are performed.

In one aspect of the invention, a method for pressure infiltrationcasting is provided including the steps of (1) providing a mold cavity,which can contain a preform, and an infiltrant charge: (2) preheatingthe mold cavity and the infiltrant charge in a heating vessel to a forma preheated mold cavity and a molten infiltrant charge; (3) transferringthe preheated mold cavity and the molten infiltrant charge to a pressurevessel;(4) pressurizing the pressure vessel so that the molteninfiltrant charge fills the preheated mold cavity and drives molteninfiltrant charge into fine details of the mold cavity to form a filledmold cavity and (5) cooling the filled mold cavity so that the molteninfiltrant solidifies to form a finished article.

In another aspect of the invention, apparatus for pressure infiltrationcasting is provided including (1) a first heating vessel for heating amold cavity, which can contain a preform, and an infiltrant charge toproduce a heated mold cavity and molten infiltrant charge and includinga chamber for containing said mold cavity and said infiltrant; (2) apressure vessel for filling the mold cavity with the infiltrant chargeunder pressure to produce a filled mold cavity; (3) a transfer chamberfor holding said mold cavity and molten infiltrant during transfer fromsaid first heating vessel to said pressure vessel; and (4) a coolingchamber for cooling said filled mold cavity to form a finished article.

It is an object of this invention to provide a method for pressureinfiltration casting according to which the mold cavity, which cancontain a preform, and infiltrant charge are heated and evacuated inseparate heating vessels from the pressure vessel wherein pressureinfiltration occurs so that a rapid throughput of finished articleslimited only by the solidification rate of the filled mold cavity and/orinfiltrated preform is achieved.

It is a further object of the invention to provide a pressureinfiltration casting apparatus which includes a separate heating vesseland pressure vessel as well as a transfer chamber for conveying theheated mold cavity and infiltrant from the heating vessel to thepressure vessel to undergo pressure infiltration therein. Thus, the moldcavity and infiltrant can be heated and infiltrated in separate vesselsso that a single multipurpose vessel is not occupied during performanceof only one of the multipurpose vessel functions so that maximumfinished article throughput, limited only by the solidification rate ofthe filled mold cavity and/or infiltrated preform, is possible.

Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art uponreading the description of the preferred embodiments which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration in cross section showing the heatingand evacuation of a preform and molten infiltrant in a vacuum furnace.

FIG. 2 is a schematic illustration in cross section showing the heatingand evacuation of a preform and molten infiltrant in a vacuum furnaceusing a vent tube.

FIG. 3 is a schematic illustration in cross section showing a system forheating and evacuating a mold containing a preform and molten infiltrantincluding a transfer container for transferring the heated and evacuatedmold to a pressure vessel.

FIG. 4 is a schematic illustration in cross section showing a system forheating and evacuating a mold containing a preform and molten infiltrantincluding a transfer container for transferring the heated and evacuatedmold to a pressure vessel.

FIG. 5 is a schematic illustration in cross section showing pressureinfiltration of a preheated, preevacuated preform in a pressure vessel.

FIG. 6 is a schematic illustration in cross section showing cooling ofan infiltrated preform in a pressure vessel.

FIG. 7 is a schematic illustration in cross section showing a heated andevacuated mold containing a preform and molten infiltrant held within avacuum furnace.

FIG. 8 is a schematic illustration in cross section of a mold containinga preheated, preevacuated preform and molten infiltrant being preparedfor transfer from a vacuum furnace to a pressure vessel.

FIG. 9 is a schematic view along the line 9--9 of FIG. 8.

FIG. 10 is a schematic illustration in cross section showing pressureinfiltration of a preheated, preevacuated preform in a pressure vessel.

FIG. 11 is a schematic illustration in cross section showing cooling ofan infiltrated preform in a pressure vessel.

FIG. 12 is a schematic illustration showing the interior of a pressurevessel and of a heating vessel including a mold vessel.

FIG. 13 is a schematic illustration in cross section showing theinterior of the mold vessel shown in FIG. 12.

FIG. 14 is a schematic illustration in cross section showing an enlargedview of a part of the interior of the mold vessel shown in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method for pressure infiltration casting of anarticle which includes steps of preheating a mold cavity, which cancontain a preform, and an infiltrant charge in a first heating vessel sothat a preheated mold cavity and molten infiltrant charge are produced,then transferring the preheated mold cavity and the molten infiltrantcharge to a pressure vessel which is pressurized so that the molteninfiltrant charge infiltrates the preheated mold cavity to produce afilled mold cavity which is cooled so that the molten infiltrantsolidifies to form a finished article.

The invention also provides an apparatus for pressure infiltrationcasting which includes a first heating vessel having a chamber whichcontains a mold cavity and an infiltrant charge for heating the moldcavity and the infiltrant charge to produce a heated mold cavity andmolten infiltrant charge; a transfer chamber for containing the heatedmold cavity and molten infiltrant charge as it is removed from the firstheating vessel and transferred to a separate pressure vessel and acooling chamber for solidifying the filled mold cavity and/orinfiltrated preform to produce a finished article.

By heating and evacuating the mold cavity, which can optionally containa preform, and the infiltrant charge in furnaces and/or vacuum furnaceswhich are separate from the pressure vessel wherein mold cavityinfiltration is performed, both the method and the apparatus of theinvention allow for rapid throughput of pressure infiltrated articleslimited only by the solidification rate of the filled mold cavity and/orinfiltrated preform. Both the method and the apparatus of the inventionprovide separate vessels wherein each of the steps of a pressureinfiltration casting process can be separately carried out by contrastwith pressure infiltration casting methods and apparatus wherein all thesteps of the pressure infiltration casting process are performedsequentially within the same multipurpose vessel which of necessitydictates that a mold cavity, which can optionally contain a preform, andinfiltrant charge cannot be heated and evacuated while an already heatedmold cavity is filled with the molten infiltrant. The method andapparatus of the invention allow for more efficient "parallelprocessing" whereby a mold cavity and an infiltrant charge can be heatedin dedicated heating vessels while another, already heated mold cavityand infiltrant charge is pressure infiltrated in a separate, dedicatedpressure vessel.

As used in this description and in the claims, the term "preform" refersto a porous body, which can include a continuous fiber reinforcement, aprimary particulate reinforcement phase, or a combination of acontinuous fiber reinforcement phase with a secondary particulatereinforcement phase which can be subsequently infiltrated to form aninfiltrated preform.

As used herein, the term "infiltration" refers to the injection underpressure of a molten liquid, the molten infiltrant charge, which can bea molten metal, metal alloy or intermetallic compound, into a moldcavity or preform under pressure.

According to the method of the invention, the heating vessel is opened,the preheated mold cavity, which can include a preform, and molteninfiltrant charge are removed from the heating vessel and transferred toan open pressure vessel which after loading is closed for pressurizingto fill or infiltrate the preheated mold cavity, which can include apreform, with the molten infiltrant charge.

Thus, using the method and apparatus of the invention, it is possible toproduce unreinforced castings having superior surface qualities, bettertolerances, thinner sections and more complex, finely detailed shapesthan can be achieved using gravity-driven casting techniques. Using thesame method and apparatus, but with the introduction of a preform intothe mold cavity, reinforced castings can also be produced.

The preform can include a continuous fiber reinforcement such as amonofilament fiber reinforcement or a multifilament tow fiberreinforcement. Typical continuous fiber reinforcement volume fractionsare in the range of from about 0.15 to about 0.85, more preferably inthe range of from about 0.30 to about 0.65 and most preferably in therange of from about 0.40 to about 0.60.

Monofilament fiber reinforcements such as slurry-spun alumina, sol-gelderived alumina, sapphire, yttrium aluminum garnet (YAG), yttria-aluminaeutectic, titanium diboride, boron nitride, boron carbide and titaniumcarbide monofilaments are suitable for use with the process of theinvention. Monofilament fiber diameters can be in the range of fromabout 50 μm to 250 μm, more preferably in the range of from about 75 μmto about 200 μm and most preferably in the range of from about 100 μm toabout 150 μm.

Suitable multifilament tow fiber reinforcements include aluminum oxide,silicon carbide, organometallic-derived silicon carbide, pitch-basedgraphitic carbon, organometallic-derived silicon nitride,polyacrylonitrile (PAN)-based, organometallic-derived titanium carbideand organometallic-derived mixed titanium carbide and silicon carbidemultifilament tow fiber reinforcements. The individual fibers of themultifilament tow can have individual fiber diameters in the range offrom about 3 μm to about 50 μm, more preferably in the range of fromabout 6 μm to about 30 μm and most preferably in the range of from about10 μm to about 20 μm.

The preform can also include a primary particulate reinforcement phasesuch as a ceramic like titanium diboride or aluminum oxide; a highmelting point metal like Mo, W, Cr, Nb or Ta; and refractory materiallike titanium diboride, aluminum oxide, yttrium oxide, boron nitride,silicon carbide, titanium carbide, zirconium carbide, hafnium carbide,tungsten carbide, niobium carbide, boron carbide, titanium nitride,zirconium nitride, hafnium nitride or diamond particulatereinforcements.

The preform can also be a hybrid preform including a secondaryparticulate reinforcement phase which can be a ceramic, high meltingpoint metal, or a refractory material, a brittle material which is notreactive with the molten infiltrant charge. Ceramic secondaryparticulate reinforcement phases can include titanium diboride andaluminum oxide. High melting point metal secondary particulatereinforcement phases can be Mo, W, Cr, Nb and Ta. Refractory materialsecondary particulate reinforcement phases can be titanium diboride,aluminum oxide, yttrium oxide, boron nitride, silicon carbide, siliconnitride, titanium carbide, zirconium carbide, hafnium carbide, tungstencarbide, niobium carbide, boron carbide, titanium nitride, zirconiumnitride and hafnium nitride. The secondary particulate reinforcementphase can be present in the interfiber spacing of the hybrid preform ata volume fraction of the interfiber spacing in the range of from about0.50 to about 0.80, more preferably in the range of from about 0.50 toabout 0.70, with the continuous fiber reinforcement being present at avolume fraction in the range of from about 0.20 to about 0.80, morepreferably in the range of from about 0.30 to about 0.70, and mostpreferably in the range of from about 0.40 to about 0.60.

The infiltrant charge can be a metal such as aluminum, silicon,magnesium, nickel, zinc, copper, iron, tin, silver, gold, platinum,rhodium, silicon, titanium, chromium, cobalt, vanadium, niobium,molybdenum, zirconium and alloys thereof, or can be an intermetalliccompound such as NiAl, Ni₃ Al, TiAl, FeAl, Fe₃ Al, CoAl and Co₃ Al.

The method of the invention can be used for preparation of continuouslyreinforced composites, particulate-reinforced and hybrid reinforcedcomposites containing the already-described reinforcements and can alsobe used to prepare bulk compounds by reactive infiltration. In areactive infiltration process, the preform is consumed by reaction withthe molten infiltrant charge to result in production of the bulkcompound. For example, a carbon preform can be infiltrated with asilicon molten infiltrant to form bulk silicon carbide.

According to one embodiment of the invention, trapped gas containedwithin the preform and the molten infiltrant charge is removed bypreheating the preform and the molten infiltrant charge in a vacuumheating vessel which is evacuated to remove the trapped gas from thepreform and the molten infiltrant charge.

According to another embodiment of the invention, a selected gasatmosphere is provided in the heating vessel wherein the preform and theinfiltrant charge are heated, such as an oxygen, ambient air, compressedair, argon, helium or nitrogen atmosphere, to protect the surfaces offibers in the preform. The molten infiltrant charge can be positionedabove the preform so that it can be poured into the preform.

After the preform and the infiltrant charge have been heated in theheating vessel in the selected atmosphere, the preheated preform and themolten infiltrant charge are removed from the heating vessel andtransferred to a vacuum vessel, which can be heated in order to retainthe molten infiltrant charge in a molten state, to remove trapped gasfrom the preform and molten infiltrant charge so that a vacuum can beisolated in the preheated preform.

FIG. 1 schematically shows the step of preheating a preform andinfiltrant charge in an inert atmosphere in a heating vessel. Furnaceelements 10 surround gas impermeable mold vessel 12 which can beconstructed of a material such as steel, quartz, alumina or other metalsor ceramics which are gas impermeable and wherein molten infiltrantcharge 14 is separated from staple preform 15 by filter 16 which can bemade of alumina staple fiber, fiberglass, mullite fiber, carbon fiber,Fiberfrax™, silicon carbide foam, carbon foam, alumina foam or zirconiafoam having an approximate porosity of from about 15% to about 85%.Usually, low volume fraction filters are used. When mold vessel 12 isplaced in a vacuum vessel for evacuation, dissolved gases within themelt and trapped within preform 15, are removed so that a vacuum isisolated in preform 15 and mold cavity 20. Gas bubbles 18 move throughthe molten metal during evacuation, resulting in evacuation of the moldcavity containing the preform. The vacuum is maintained because a vacuumseal is established at surface 22 where molten infiltrant charge 14meets surface 24 of mold vessel 12. Typical vacuum levels attained inthe vacuum vessel are in the range of from about 10 μm Hg to about 1 mmHg.

The preform and mold cavity can also be evacuated by positioning a venttube in the molten infiltrant charge and in fluidic contact with thepreform as schematically depicted in FIG. 2. In FIG. 2, vent tube 26 ispositioned in filter 16 so that it terminates within filter 16. Venttube plug 17 made of fiber can be made of the same or similar materialas that used to make filter 16 and prevents molten infiltrant chargefrom entering vent tube 26 and from thence the vacuum apparatus. End 27of vent tube 26 is connected to a vacuum apparatus not shown. Vent tube26, which can be made of steel, quartz, alumina or any other gasimpermeable material chemically compatible with the molten infiltrantand preform can be positioned at the top of preform 15 when theinfiltrant charge 14 and preform 15 are being loaded in mold vessel 12or can be positioned after infiltrant charge 14 is molten and preform 15is preheated. Vent tube 26 is removed before transfer of preform 15 andmolten infiltrant charge 14 into the pressure vessel.

According to another embodiment of the invention, the infiltrant chargeand the preform are heated separately with the infiltrant charge beingheated in an infiltrant heating vessel to form a molten infiltrantcharge and the preform being heated in a preform heating vessel, whichcan include vacuum apparatus for evacuating the preform, to form apreheated preform. The preheated preform and the molten infiltrantcharge can be brought into contact, such as by introducing the molteninfiltrant charge into the preform heating vessel and pouring the molteninfiltrant into the preform which can include a vent tube, with optionalevacuation of the infiltrant-containing preform before transfer of thepreheated preform and molten infiltrant charge to the pressure vessel.The pressure vessel can be heated and can be in thermal contact with thepreheated preform and the molten infiltrant charge so that the molteninfiltrant charge remains liquid during the pressurizing step.Alternatively, the pressure vessel can be maintained at ambienttemperature (i.e., the temperature of the environment outside thepressure infiltration casting apparatus), and the preheated preform andmolten infiltrant charge kept insulated from the pressure vessel so thatthe molten infiltrant charge remains liquid through the pressurizingstep (4).

As shown schematically in FIGS. 3 and 4, an insulated or heated transfercontainer can be provided for transferring the preheated preform and themolten infiltrant charge from the heating vessel to the pressure vesselso that the molten infiltrant charge remains molten and the preheatedpreform does not cool.

FIGS. 3 and 4 schematically depict the loading of a preevacuated,preheated preform and molten infiltrant charge into a transfer chamberfor removal from a vacuum heating vessel and transfer to a pressurevessel.

In FIG. 3, preheated preform 15 is separated from molten infiltrantcharge 14 by filter 16 within mold vessel 12 so that a vacuum isisolated in mold cavity 20 and preform 15. Mold vessel 12 is equippedwith an insulating cap 30 which can be made of a refractory materialfiber such as alumina fiber, Saffil™, Fiberfrax™ or zirconia wool and ispositioned between furnace elements 10 to keep preheated preform 15 atan appropriate elevated temperature and retain molten infiltrant charge14 in the molten state. Mold vessel 12 rests on hydraulic extractor 32which is typically a steel rod which can be withdrawn from vacuum belljar 34 at vacuum bell jar outlet 36. Mold vessel 12 is thermallyinsulated from hydraulic extractor 32 by insulator plate 38 typicallymade of alumina-silicate fiber such as Fiberfrax™ produced byCarborundum Co., spun-alumina staple fiber such as Saffil™ produced byICI, Co. or zirconia wool. When hydraulic extractor 32 is moved downwardin the direction given by arrow 40, mold vessel 12 can be positioned inmold transfer chamber 42 made of mold transfer chamber outer walls 43typically constructed of a metal such as steel, approximately 0.030inches thick, mold transfer chamber inner walls 44 typically constructedof an insulating material 45 such as alumina fiber, Saffil™, Fiberfrax™or zirconia wool approximately one inch thick and mold transfer chamberbaseplate 41 constructed of alumina fiber, Saffil™, Fiberfrax™ orzirconia wool insulating material 0.5 inches thick. Alternatively, moldtransfer container inner walls 44 can also include electric resistanceheaters not shown constructed of an Fe-Cr alloy; Nichrome™, a Ni-Cralloy material; Kanthal™, an FeCoAlY alloy, SiC, or SuperKanthal™, aMoSi₂ material to maintain the temperature of the preform and molteninfiltrant charge at a temperature above the liquidus temperature of theinfiltrant charge. The mold vessel can also be heated by induction.

FIG. 4 shows mold transfer container 42 equipped with a bail attachmentand release mechanism 46, including primary suspension rods 47,secondary suspension rods 49 and bail primary suspension hook 51.

After mold transfer container 42 has been removed from vacuum bell jar34, mold transfer container 42 which maintains the temperature of themolten infiltrant charge and the preform at a desired level can beloaded into pressure vessel 50 and can be attached inside pressurevessel 50 to pressure vessel bail attachment hook 52 as shownschematically in FIG. 5. Insulator plate 38 remains attached tohydraulic extractor 32 so that in subsequent processing steps, thebottom of mold vessel 12 is not insulated. Pressure vessel 50 istypically constructed of a material such as steel or stainless steel.

A pressurized gas such as nitrogen, argon or helium is then introducedinto pressure vessel 50 from a gas reservoir not shown throughpressurized gas inlet 54, typically at a pressure in the range of fromabout 3 atm to about 1500 atm, more preferably in the range of fromabout 20 atm to about 500 atm, and most preferably in the range of fromabout 50 atm to about 150 atm which is sufficient to force molteninfiltrant charge 14 into preform 15 sufficiently rapidly so that theprocess remains economically feasible. Since a vacuum was originallyisolated in mold cavity 20, a pressure differential is created betweenthe isolated vacuum in mold chamber 12 and interior 55 of pressurevessel 50 containing pressurized gas 56 sufficient to cause molteninfiltrant charge 14 to be forced through filter 16 to infiltratepreform 15 quite rapidly, in a time period which can vary from about afraction of a minute to on the order of minutes.

Once infiltration of preform 15 by molten infiltrant charge 14 hasproceeded to the desired extent as evidenced by processing experienceand calculations as well known to one skilled in the art, hydraulicextractor 32 is moved so that it comes into contact with the bottom ofmold vessel 12. Then, bail attachment and release mechanism 46 isreleased so that mold vessel 12 can be lowered into chill zone 58 oronto a chill plate not shown. In yet another embodiment, not shown,extractor 32 can be water-cooled so that it provides the requiredsolidification gradient when in thermal contact with mold vessel 12.

Excess molten infiltrant charge 56 which does not enter preform 15 forms"hot top" 57 above preform 15 as shown in FIG. 6. "Hot top" 57 insuresthe proper temperature gradient such that metal shrinkage due tosolidification is continuously replenished from the pool of molten metalin the "hot top" 57. With shrinkage thus fed, shrinkage porosity withinthe preform cavity is eliminated. The solidification gradient isselected and maintained so that shrinkage porosity is eliminated. Thepresence of "hot top" 57 provides the required thermal gradient toinitiate directional solidification of infiltrated preform 15 as moldvessel 12 is lowered into chill zone 58 as extractor mechanism 32 ismoved downward in the direction given by arrow 60 so that secondarysuspension rods 49 are disengaged from primary suspension rods 47 atbail secondary suspension hook 39.

Chill zone 58 can be constructed from hydraulically cooled metal platesin thermal contact with mold vessel 12 and refers to the section ofpressure vessel 50 where directional solidification of the infiltratedpreform occurs. Typical pressures maintained in the pressure vesselduring infiltrated preform solidification are in the range of from about50 atm to about 125 atm. Typical temperature gradients used to achievedirectional solidification of infiltrated preform 15 are in the range offrom about 550° C. to about 650° C. for aluminum alloys. Alternatively,chill zone 58 can include heaters which can be adjusted to control chilltemperature and to achieve a selected solidification rate in infiltratedpreform 15 as determined by considering the following characteristics ofthe material being solidified: interfiber spacing within preform 15,infiltrant solidus temperature and infiltrant liquidus temperature.

Also, solidification can be conducted in a chill vessel having any orall of the chill zone components already described which is separatefrom the pressure vessel.

Solidification can also be conducted by flowing a gas around theinfiltrated preform. The gas can be the same gas as used to drive theinfiltrant into the preform when gas pressure is at a level sufficientto cause infiltration.

The pressure infiltration casting apparatus of the invention provides aseparate heating vessel to heat a preform and an infiltrant charge toproduce a preheated preform and a molten infiltrant charge, a separatepressure vessel to infiltrate the preform with a molten infiltrantcharge to produce an infiltrated preform, a transfer chamber to hold thepreheated preform and molten infiltrant charge during transfer from theheating vessel to the pressure vessel and a cooler to cool theinfiltrated preform to form a finished article. In a preferredembodiment of the apparatus, the heating vessel also includes vacuumequipment to evacuate the heating vessel and remove trapped gas from thepreform and the molten infiltrant charge.

In another preferred embodiment, the heating vessel contains a selectedatmosphere which can be an inert gas such as argon, nitrogen or heliumor a reducing gas, if the infiltrant charge is copper or steel, orambient atmosphere (air). In this embodiment, a vacuum vessel isprovided to evacuate the preheated preform and molten infiltrant chargeto remove any gases which are trapped within the preheated preform andmolten infiltrant.

The vacuum vessel can include heaters and be in thermal contact with themold vessel containing the preheated preform and molten infiltrant.Alternatively the vacuum vessel can be unheated and can be insulatedfrom the mold vessel containing the preheated preform and molteninfiltrant charge. The vacuum vessel can include a vent tube which isinserted near a surface of the preform so that the vent tube is influidic contact with the preform. A plug of refractory fibrous materialcan be inserted in the vent tube to prevent molten infiltrant chargefrom being drawn up into the vacuum equipment during evacuation of thepreform. The vent tube can be connected to the vacuum equipment forremoval of trapped gases from within the preheated preform and molteninfiltrant charge. The vent tube can be constructed from materials suchas, but not restricted to, steel, ceramics such as alumina, mullite orzirconia, quartz or glass. A vacuum in the range of from about 1 μm Hgto about 1 mm Hg is usually sufficient for evacuation of the preheatedpreform and molten infiltrant charge.

Optionally, the infiltrant charge can be held in a container within theheating vessel and the molten infiltrant charge can be poured from thecontainer into the preheated preform.

In another embodiment, separate preform and infiltrant heating vesselsare provided and the preform heating vessel can further include vacuumequipment to evacuate the preform.

The pressure vessel can be provided with a heating device to heat thepreheated preform and keep the molten infiltrant charge in a liquidstate. The pressure vessel can alternatively, be kept at ambienttemperature and provided with insulation to insulate the pressure vesselfrom the preheated preform and molten infiltrant charge. The pressurevessel is provided with a gas inlet for introduction of pressurized gasfor infiltration and is constructed from material such as steel towithstand gas pressures in the range of from about 1 atm to about 1500atm, more preferably in the range of from about 20 atm to about 500 atm,and most preferably in the range of from about 50 atm to about 150 atm.Approximately 1 atm to about 5 atm of pressure are sufficient forinfiltration of unreinforced castings and for filling complex moldsincluding fine details.

A transfer chamber is provided to transfer the preheated preform andmolten infiltrant from the heating vessel to the pressure vessel and caninclude insulation to prevent the preheated preform and molteninfiltrant charge from cooling during transfer from the heating vesselto the pressure vessel or can be equipped with an electrical resistanceheater to maintain the preheated preform and molten infiltrant charge ata desired temperature.

The transfer chamber can be constructed as a bail transfer chamberincluding a mold vessel holding chamber to contain the mold vessel, aheating vessel suspension member, a pressure vessel suspension member,primary suspension rods connected to the mold vessel chamber todetachably suspend the mold vessel holding chamber from the heatingvessel and pressure vessel suspension members, and secondary suspensionrods connected to the mold vessel to detachably suspend the mold vesselfrom the primary suspension members as shown in FIG. 6.

Alternatively, the transfer chamber can be a common transfer headtransfer chamber as shown in FIG. 7. A common transfer head moldtransfer chamber can be used to transfer the preheated, preevacuatedpreform and molten infiltrant charge from the heating or vacuum heatingvessel to the pressure infiltration vessel as shown schematically inFIGS. 7-10 and can be insulated or include heaters to maintain thepreheated preform and molten infiltrant charge at a desired temperature.

FIG. 7 shows vacuum heating vessel 70 which contains mold vessel 12suspended from transfer cap 71, which can contain instrumentationincluding thermocouples and pressure gauges, by common transfer headsuspension chamber 72 and be made of metal or insulating material.Common transfer head suspension chamber 72 includes support members 61which suspend mold vessel 12 during heating and/or evacuation and can berods or bars which can be rotated out of the way as common transfer headmold transfer chamber 76 is raised to allow mold vessel 12 to slide intotransfer chamber 76. Mold vessel 12 contains preform 15 separated byfilter 16 from infiltrant charge 14 and is covered by insulator cap 30.Common transfer head suspension chamber 72 holds mold vessel 12 and itscontents within a space enclosed by multizone furnace elements 10 whichpreheat preform 15 and melt infiltrant charge 14 so that it becomes amolten infiltrant charge.

Once vacuum heating vessel 70 is evacuated, preform 15 is evacuated anda vacuum is isolated in mold vessel cavity 20. Vacuum heating vessel 70also includes preheat furnace elements 74 for preheating common transferhead mold transfer chamber 76.

Common transfer head mold transfer chamber 76 includes common transferhead mold transfer chamber outer walls 78 which are typically made froma metal such as steel or stainless steel capable of withstanding castingprocessing temperatures typically in the range of from about 660° C. toabout 750° C. and common transfer head mold transfer chamber inner walls80 which can be constructed from an insulating material such as aluminafiber, Fiberfrax™, Saffil™ or zirconia wool having adequate insulatingproperties to maintain the preheated preform molten infiltrant charge ata temperature above the liquidus of the infiltrant charge and aretypically in the range of from about 0.5 inch to about 2 inches thick.Chamber inner walls 80 can also include heaters to maintain preheatedpreform 15 and molten infiltrant charge 14 at a desired temperature.

Common transfer head mold transfer chamber baseplate 82 is typicallyconstructed from a material such as steel or stainless steel and ismounted on hydraulic common transfer head mold transfer chamber transferrod 84. Transfer chamber baseplate 82 can be removably mounted ontransfer rod 84 in any manner known to one skilled in the art includinghaving one or more pins protrude from the bottom of transfer chamberbaseplate 82 which are inserted into mating holes in transfer rod top 83when transfer chamber baseplate 82 is attached to transfer rod top 83.When transfer chamber baseplate 82 is attached to transfer rod top 83,transfer rod 84 can be used to lift common transfer head mold transferchamber 76 in the direction given by arrow 86 as shown in FIG. 8 so thatcommon transfer head mold transfer chamber 76 engages common transferhead suspension chamber 72 using a bayonet mechanism whereby outerbayonet mechanism slots 73 engage mating support tabs on transferchamber baseplate 82 when transfer rod 84 is appropriately rotated by60° in the direction given by arrow 87.

FIG. 9 show a view of bayonet mechanism 91 taken along line 9--9 of FIG.8. In FIG. 9, bayonet mechanism 91 is shown in the engaged position whensupport tabs 85 which are machined into transfer chamber baseplate 82are rotated so that they are displaced from outer bayonet mechanismattachment slots 73. When bayonet mechanism 91 is in the engagedconfiguration as shown in FIG. 9, transfer chamber baseplate 82 isconnected with common transfer head suspension chamber 72. Transfer rod84 can be rotated by 60° so that slots 73 and tabs 85 are alignedthereby disengaging transfer chamber baseplate 82 from common transferhead suspension chamber 72.

Mold transfer chamber 76 maintains molten infiltrant charge 14 andpreheated preform 15 at a desired temperature until preheated preform 15is infiltrated with molten infiltrant charge 14 by application ofpressure.

Using hydraulic common transfer head mold transfer chamber transfer rod84, common transfer head mold transfer chamber 76 enclosing commontransfer head suspension chamber 72 and mold vessel 12 can betransferred to pressure vessel 90 as shown in FIG. 10 where commontransfer head mold transfer chamber 76 is engaged with common transferhead suspension chamber 72 whereupon transfer rod 84 can be disengagedfrom common transfer head mold transfer chamber baseplate 82.Pressurization gas such as nitrogen at a pressure of typically 100 atmcan be introduced into pressure vessel 90 through pressurization gasinlet 92 from a pressurization gas reservoir not shown to create apressure differential between the pressure inside pressure vessel 90 andthat in mold cavity 20 wherein a vacuum has been isolated by evacuatingmold vessel 12 in vacuum heating chamber 70 resulting in infiltration ofpreheated preform 15 with molten infiltrant charge 14.

The quantity of the metal charge is calculated so that the proper amountof molten charge remains as hot top 94 after preform 15 and mold cavity20 have been completely infiltrated. Most preforms that have beenpreheated to a temperature near to the liquidus temperature of themolten infiltrant charge are infiltrated in a period of time typicallyless than one minute.

Once infiltration has proceeded to the desired level, leaving anappropriate amount of residual molten infiltrant charge 14 to produce"hot top" 94, hydraulic common transfer head mold transfer chambertransfer rod 84 which can be water-cooled once again can be engaged withtransfer chamber baseplate 82 so that transfer rod 84 is in thermalcontact with mold vessel 12 and infiltrated preform 15 so that anappropriate temperature gradient for directional solidification isestablished.

Alternatively, as shown in FIG. 11, mold vessel 12 containinginfiltrated preform 15, filter 16 and "hot top" 94 can be released fromcommon transfer head suspension chamber 72 such as by moving awaysupport members not shown in such a way as to keep transfer chamber 76suspended from transfer cap 71 by common transfer head suspensionchamber 72. Mold vessel 12 is then withdrawn in the direction given byarrow 96 into chill zone 98 so that directional solidification ofinfiltrated preform 15 can occur as shown in FIG. 11.

Chill zone 98 can be constructed from chill plates 100 made from highlythermally conductive material such as metals like copper, molybdenum,tungsten, or steel or non-metals such as graphite. Chill plates 100 canbe optionally provided with internal fluid circulation channels notshown so that a cooled fluid can be continuously circulated throughchill plates 100 to maintain chill plates 100 at a selected temperatureand continually carry away the heat of solidification from solidifyinginfiltrated preform 15. Chill plates 100 can also be provided with acooling chamber heater not shown which can be used to control thetemperature of chill zone 98 and produce a controlled, selectedsolidification rate. A lower chill baseplate 102 is attached to and inthermal contact with transfer rod support 104 which allows heat transferfrom mold vessel base 106. Lower chill baseplate 102 can bewater-cooled. Lower chill baseplate 102 can be used alone or togetherwith chill plates 100, depending upon the desired temperature gradientfor solidification. Lower chill baseplate 102 can be connected totransfer rod 84 and be in thermal contact therewith. The chill baseplate102 can be brought into contact with the infiltrated preform either byraising the chill baseplate 102 up to the infiltrated preform or bylowering the infiltrated preform onto chill baseplate 102. Also,transfer rod 84 can be watercooled.

In another embodiment, solidification is carried out in a chill vessel,separate from the pressure vessel and equipped with thealready-described components of chill zone 98. Solidification of theinfiltrated preform can also be conducted by chilling the infiltratedpreform on a chill plate exterior to the pressure vessel.

In another preferred embodiment, as shown schematically in FIG. 12,heating vessel 110, which can be a vacuum or controlled atmospherefurnace, and separate pressure vessel 112 are arranged in a horizontalconfiguration and are connected to each other by rail transfer system114. Rail transfer system 114 allows transfer of mold vessel 116 fromheating vessel 110 to pressure vessel 112 in a continuous fashion andallows for a continuous high volume cycling of mold vessels and preformsthrough a pressure infiltration process.

Mold vessel 116 contains a mold, a preform and molten infiltrant chargeand serves as a transfer chamber for transfer of the mold and preformfrom heating vessel 110 to pressure vessel 112 and moves along, railtransfer system 114 on shoes 115. Mold vessel 116 can be preheated andpreevacuated in heating vessel 110, optionally using one or more venttubes not shown for evacuation. Once evacuation is complete, any venttubes are removed and mold vessel 116 is transferred from heating vessel110 to pressure vessel 112 using rail transfer system 114.

Once mold vessel 116 is introduced into pressure vessel 112, pressurizedgas is introduced into pressure vessel 112 so that molten infiltrant118, contained within mold vessel 116 as shown in the schematic interiorview of mold vessel 116 of FIG. 13, infiltrates preform 119 alsocontained within mold vessel 116 and shown in FIG. 12. Afterinfiltration is complete, mold vessel 116 is brought into contact withchill block 126 which is water cooled by water circulation apparatus128. Since chill block 126 is in contact with an entire side of moldvessel heat transfer is efficient.

Mold vessel 116 can be brought into contact with chill block 126contained in pressure vessel 112 in several ways to solidify theinfiltrated preform. Mold vessel 116 can be slid off rail transfersystem 114 onto chill block 126 by providing a downgrade from heatingvessel 110 and pushing mold vessel 116 off the rail transfer system 114and onto chill block 126. Also, chill block 126 can be raised therebylifting mold vessel 116 off rail system 114 to bring it into contactwith mold vessel 116. Alternatively, mold vessel 116 can be in contactwith chill block 126 from the beginning of the infiltration process andinfiltrated with the molten infiltrant charge while in thermal contactwith chill block 126.

FIG. 13 is a schematic cross-sectional representation of the interior ofmold vessel 116 during the heating and evacuation step and showsinsulated mold vessel outer walls 130, 132 and 134 including upperinsulating layer 131 surrounding thermally conductive mold vessel innerwalls 136, 137 and 138 which can be steel. Mold vessel 116 isconstructed so that its walls are thermally conductive and the moldvessel is gas impermeable. A fibrous layer 140 can be positioned betweenvessel inner wall 137 and mold 120 and can be made of the same fibrousinsulating material as filter 142. During heating in heating vessel 110and prior to infiltration in pressure vessel 112, fibrous layer 140 actsas an insulator. During infiltration, fibrous layer 140 is alsoinfiltrated with molten infiltrant 118 and, thus, is made thermallyconductive if molten infiltrant 118 is a metal or other thermallyconductive material. Once infiltrated, fibrous layer 140 allows heattransfer between mold 120 and chill block 126.

FIG. 14 is an expanded cross-sectional, schematic view of a section ofthe mold vessel interior shown schematically in FIG. 13 and additionallyshows preforms 150 contained within mold 120 fed with molten infiltrantthrough preform gates 152 from sprues 154. Vacuum vent tube 156 isembedded within filter 142 for evacuating mold 120 and preforms 150.Multiple vacuum vent tubes can also be provided.

It is emphasized that all of the foregoing methods and apparatus can beused to produce an unreinforced casting by eliminating the preform and,instead, using an empty mold cavity having the shape of the desiredcasting.

In order to further illustrate the method and apparatus of the presentinvention, the following Examples are provided. The particularprocessing conditions and design details of the apparatus utilized inthe Examples are meant to be illustrative of the present invention andnot limiting thereto.

EXAMPLE 1

The following Example is provided to show how a preform can bepreevacuated before being infiltrated to result in production of afinished composite article without any porosity due to residual gas inthe preform.

A 1.5 inch diameter quartz mold vessel was loaded with a SiC powderpreform which filled the bottom four inches of the quartz mold vessel. Afilter material such as any of those already described was placed on topof the SiC powder preform and a solid infiltrant charge of solidaluminum was placed above the filter material.

The loaded mold vessel was then heated in an inert atmosphere such asany of those inert gas atmospheres already described until the solidaluminum solid infiltrant charge melted to form a molten infiltrantcharge of molten aluminum and the SiC powder preform was preheated tothe aluminum infiltrant charge liquidus temperature.

The mold vessel containing the molten aluminum molten infiltrant chargeand the preheated SiC powder preform was then evacuated to cause any gasentrapped in the SiC powder perform to be removed by bubbling throughthe molten aluminum molten infiltrant charge. Evacuation was carried outfor approximately 10 minutes.

After mold vessel evacuation was complete, the vessel containing thequartz vessel was pressurized to cause the molten aluminum molteninfiltrant charge to infiltrate the preform.

After infiltration was completed to the desired level, the infiltratedpreform was solidified to produce a finished casting.

The casting was inspected for porosity and completeness of infiltrationand was found to be sound and completely infiltrated with no evidence ofresidual gas porosity indicating that any entrapped gas had beencompletely removed during the evacuation step conducted prior toinfiltration.

EXAMPLE 2

The following Example is provided to show how a vent tube can be used toevacuate a preform during an evacuation step of a pressure infiltrationprocess to produce a finished article which shows no evidence ofporosity due to residual trapped gas.

A quartz mold vessel was loaded with a SiC 600 grit powder preform. Afilter made up of a 0.75 inch thickness of Fiberfrax™ material wasplaced on top of the SiC powder preform. A quartz vent tube (having 6 mmi.e. 0.25 inch inner diameter) was inserted into the filter material sothat its end nearest the preform surface was approximately 0.25 inchfrom the preform surface as schematically shown in FIG. 2. The other endof the quartz tube was connected to a vacuum. The quartz vent tube waspacked with a 0.5 inch plug, also of Fiberfrax™ material, to preventmolten infiltrant charge from being pulled up through the tube and intothe vacuum pump during subsequent evacuation steps. A solid aluminumsolid infiltrant charge was placed above the filter and around thequartz vent tube.

The loaded mold was then heated in an argon atmosphere to a temperatureabove the liquidus temperature of the aluminum solid infiltrant charge.

After the solid aluminum solid infiltrant charge was fully melted toform a liquid aluminum molten infiltrant charge, the quartz vent tubewas opened to vacuum using a valve so that the preform and mold vesselwere evacuated. After evacuation of the preform and mold vessel throughthe vent tube for approximately 10 minutes, the vent tube was manuallyremoved from the preform and mold vessel.

The preheated preform and molten infiltrant charge were then pressurizedby exposure to an 800 psi nitrogen atmosphere to achieve completeinfiltration of the SiC powder preform. The infiltrated preform wassolidified to produce a finished casting.

Examination of the finished casting showed that the SiC powder preformhad been completely infiltrated and showed no evidence of porosity dueto any residual trapped gas.

What is claimed is:
 1. A method for pressure infiltration castingcomprising:(1) providing a mold having a mold cavity including a preformand an infiltrant charge; (2) preheating said mold, said preform, andsaid infiltrant charge in a heating vessel to form a preheated mold,preform, and molten infiltrant charge; (2a) transferring said preheatedpreform and said molten infiltrant charge from said heating vessel to avacuum vessel which is evacuated to isolate a vacuum in said preheatedpreform; (3) transferring said preheated mold, said preform, and saidmolten infiltrant charge to a pressure vessel; (4) pressurizing saidpressure vessel so that said molten infiltrant charge fills saidpreheated mold cavity to form a filled mold cavity and an infiltratedpreform; (4a) positioning a vent tube in fluidic contact with saidpreheated preform and removing said vent tube before step (3) oftransferring said preheated preform and said molten infiltrant chargefrom said heating vessel to said pressure vessel; (5) cooling saidfilled mold cavity and said infiltrated preform so that said molteninfiltrant solidifies to form a finished article.
 2. The method of claim1 wherein said preform includes a continuous fiber reinforcementselected from the group consisting of a monofilament fiber reinforcementand a multifilament fiber reinforcement tow.
 3. The method of claim 2wherein said fiber reinforcement is a monofilament fiber reinforcementselected from the group consisting of slurry-spun alumina, sol-gelderived alumina, sapphire, yttrium aluminum garnet (YAG), yttria-aluminaeutectic, titanium diboride, boron nitride, boron carbide and titaniumcarbide monofilament fiber reinforcements.
 4. The method of claim 2wherein said multifilament tow fiber reinforcement is a multifilamenttow fiber reinforcement selected from the group consisting of aluminumoxide, silicon carbide, organometallic-derived silicon carbide,pitch-based graphitic carbon, organometallic-derived silicon nitride,polyacrylonitrile(PAN)-based, organometallic-derived titanium carbideand organometallic-derived mixed TiC and SiC multifilament tow fiberreinforcements.
 5. The method of claim 1 wherein said preform includes aprimary particulate reinforcement phase.
 6. The method of claim 5wherein said primary particulate reinforcement phase is a high meltingpoint metal selected from the group consisting of Mo, W, Cr, Nb, and Ta.7. The method of claim 5 wherein said primary particulate reinforcementphase is a refractory material selected from the group consisting oftitanium diboride, aluminum oxide, yttrium oxide, boron nitride, siliconcarbide, silicon nitride, titanium carbide, zirconium carbide, hafniumcarbide, tungsten carbide, niobium carbide, boron carbide, titaniumnitride, zirconium nitride, hafnium nitride and diamond.
 8. The methodof claim 2 wherein said preform further includes a secondary particulatereinforcement phase and is a hybrid preform.
 9. The method of claim 8wherein said secondary particulate reinforcement phase is a high meltingpoint metal selected from the group consisting of Mo, W, Cr, Nb and Ta.10. The method of claim 8 wherein said secondary particulatereinforcement phase is a refractory material selected from the groupconsisting of titanium diboride, aluminum oxide, yttrium oxide, boronnitride, silicon carbide, silicon nitride, titanium carbide, zirconiumcarbide, hafnium carbide, tungsten carbide, niobium carbide, boroncarbide, titanium nitride, zirconium nitride, hafnium nitride anddiamond.
 11. The method of claim 1 wherein said molten infiltrant ispositioned above said preform and flows into said preform.
 12. Themethod of claim 1 wherein said preform and said molten infiltrant chargecontain trapped gas and step (2) of preheating said preform and saidinfiltrant charge further includes a step of evacuating said heatingvessel to remove said trapped gas from said preform and said molteninfiltrant charge.
 13. The method of claim 1 wherein step (2) furtherincludes a step of providing a selected gas atmosphere in said heatingvessel.
 14. The method of claim 13 wherein said selected gas atmosphereis selected from the group consisting of argon, nitrogen, oxygen, areducing gas, ambient air, compressed air and helium gas atmosphere andmixtures thereof.
 15. The method of claim 1 wherein said vent tube ispositioned before step 2 of preheating.
 16. The method of claim 1wherein said vent tube is positioned after step (2) of preheating. 17.The method of claim 1 wherein in step (2) said infiltrant charge isheated in an infiltrant heating vessel to form said molten infiltrantcharge and said preform is heated in a preform heating vessel to form apreheated preform.
 18. The method of claim 17 wherein said preformheating vessels includes vacuum equipment for evacuating said preform.19. The method of claim 18 further comprising a step of exposing saidmolten infiltrant charge to said preheated preform before step (3) oftransferring.
 20. The method of claim 1 wherein said pressure vessel isheated and is in thermal contact with said preform and said molteninfiltrant charge so that said molten infiltrant charge is a liquiduntil step (5) of cooling said infiltrated preform.
 21. The method ofclaim 1 wherein said pressure vessel is maintained at ambienttemperature and said preform and said molten infiltrant charge areinsulated from said pressure vessel so that said molten infiltrant is aliquid until step (5) of cooling said infiltrated preform.
 22. Themethod of claim 21 wherein step (3) further includes a step of providingan insulated transfer container for transferring said preform and saidmolten infiltrant charge to said pressure vessel.
 23. The method ofclaim 21 wherein step (3) further includes a step of providing a heatedtransfer container for transferring said preform and said molteninfiltrant charge to said pressure vessel.
 24. The method of claim 1wherein step (4) of pressurizing said pressure vessel is conducted at apressure selected to create a pressure differential sufficient to causesaid molten infiltrant charge to flow into said preform.
 25. The methodof claim 24 wherein said pressure is in the range of from about 1 atm toabout 1500 atm.
 26. The method of claim 1 wherein step (5) of coolingsaid infiltrated preform further includes steps of removing saidinfiltrated preform from said pressure vessel to a chill vessel so thatsaid molten infiltrant solidifies at a selected solidification rate. 27.The method of claim 1 wherein step (5) of cooling said infiltratedpreform further includes steps of providing a chill zone in saidpressure vessel and withdrawing said infiltrated preform into said chillzone so that said molten infiltrant solidifies at a selectedsolidification rate.
 28. The method of claim 1 wherein step (5) ofcooling said mold cavity wherein said mold cavity is furthercharacterized by a mold cavity bottom further includes flowing a gas tocontact said mold cavity bottom so that said molten infiltrantsolidifies at a selected solidification rate.
 29. The method of claim 27further including a step of controlling said selected solidificationrate by providing a hot top on said infiltrated preform.
 30. The methodof claim 24 wherein said pressure is in the range of from about 20 atmto about 500 atm.
 31. The method of claim 24 wherein said pressure is inthe range of from about 50 atm to about 150 atm.
 32. The method of claim1 wherein said molten infiltrant charge reacts with and consumes saidpreform to form a bulk compound according to a reactive infiltrationprocess.